1. The problem statement, all variables and given/known data A capacitor of capacitance C is charged to a potential difference v from the cell and then disconnected from it. A charge Q is now given to its positive plate. The potential difference across the capacitor is now? 2. Relevant equations 3. The attempt at a solution V is the potential difference. Charge +Q is given and hence -Q is induced on the negative plate. The potential difference should be V+Q/C But the answer is V+Q/2C. Why?